Can I lower High colesterol levels without medication? Just fish oil and healthy diet?

Possible. About 75% of your cholesterol is dependent upon genetics. So if your cholesterol is modestly elevated,change in your diet and excercise may do the trick. Fish oil may impact your triglycerides but has no significant effect on cholestrol. Statins are drug of choice.
Misinformed? Not likely. Most cholesterol issues are genetic caused problems. They are not lifestyle or diet caused problems.